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The laboratory is responsible for providing the UK's national time scale - known as UTC(NPL) - which is controlled by atomic clocks that are among the most accurate in the world.

Peter Whibberley, senior research scientist with NPL's time and frequency group, said: "Atomic clocks are more than a million times better at keeping time than the rotation of the Earth, which fluctuates unpredictably.

"Leap seconds are needed to prevent civil time drifting away from Earth time. Although the drift is small - taking around 1,000 years to accumulate a one-hour difference - if not corrected it would eventually result in clocks showing midday before sunrise."

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Leap seconds can be a headache for communication networks, financial systems and other applications that rely on precise timing, and must be programmed into computers to avoid mistakes.

The extra seconds are introduced in the last minute of either December or June, or rarely March or September.

It is also possible for a second to be removed from the UTC (Universal Co-ordinated Time) timescale, although this has never happened.

The International Earth Rotation and Reference Systems Service (IERS), based at the Paris Observatory in France, tracks the Earth's rotation and announces when a leap second is needed roughly six months in advance.